Most normal, non-energy-efficient, vehicle and building windows use a single glass pane that readily conducts heat. Normal windows transfer energy by (1) non-solar heat gain by direct conduction, convection, and radiation through the glass; (2) solar heat gain in the form of radiation; and (3) airflow from both ventilation and infiltration through the glass. There are also commercially available window technologies with increased energy benefits that control heat intrusion into vehicles and buildings. One type of energy-efficient window has multiple panes, for example, double, triple, or quadruple panes, of glass that minimize heat transfer with insulating air spaces between the panes. Efficiency is further increased when the insulating spaces are filled with nontoxic gases such as krypton or argon, and when low-conductivity pane spacers are used between the panes. Another way of decreasing heat transfer through glass is to apply a low-emissivity, or low-e, coating to the glass surface. A low-e coating is typically a microscopically thin metal or metallic oxide layer that is applied to an outside glass pane. The low-e coating inhibits radiation from passing through the glass. The low-e coating also keeps buildings cooler in the summer by reflecting sunlight. In the winter, low-e coatings on interior panes maintain indoor temperatures by preventing heat from escaping. Other ways of decreasing heat transfer through glass include the application of a colored, dyed, or reflective film that is affixed to glass. Related art energy-efficient windows and energy efficiency ratings are described in “Selecting Windows for Energy Efficiency”, US Department of Energy Publication, DOE/GO-DE-AC03-76SF00098 PUB-788 January 1997—5000.
The heat gain through a window is typically measured by energy performance characteristics such as U-factor, R-value, Solar Heat Gain Coefficient (SHGC), and Visible Light Transmittance (VT or VLT). Low-e coatings reflect heat back into the home during cold weather and back to the outdoors during warm weather, and lower the U-factor of a window, but are not as efficient at insulating as multiple pane windows. Multiple pane windows are subject to loss of insulating gas and decreased energy efficiency over time. Colored, dyed, or reflective windows are limited in the amount of light that is let into a room (visible light transmittance). And window films with a fixed tint, while desired by users during bright sunlight days, are undesirable on cloudy days and in the evenings. Tinted windows are also architecturally undesirable in many applications.
Accordingly, it would be advantageous to provide thermally efficient window films and windows that transmit a maximum fraction of incident visible light intensity, while blocking the radiant energy of the sun and insulating against thermal energy transfer. It would also be advantageous to provide window films and windows for cold climates that have a solar heat gain coefficient that maximizes heat gain during winter, with a U-factor which reduces conductive heat transfer, and a high visible transmittance for good light transfer. In addition, it would be advantageous to provide a film that can be applied to a regular window, thereby converting the window into an energy-efficient window. Further, it would be advantageous for the film to be flexible, such that it can bend to accommodate non-planar surfaces.